So which tests are accredited by the Civil Aviation Authorities of the Sultanate of Oman?
It is not good enough to submit a certificate from any old test that describes the levels in each skill and a certificate that the raters have passed a tester/rater course "which complies with ICAO criteria" (who is going to certify that since ICAO neither approves nor endorses tests or raters?).
The CAAs of this world have been remise in not insisting that ICAO rationalises the LPR tests and ensures reciprocal recognition of tests (like type ratings).
The whole aim of the ICAO LPRs was to improve safety by improving communication. The poor pilot and ATC who has to comply is left with a can of worms created by the CAAs of each country, and ICAO who they are all members of and who created a wishy-washy system. This is proven by the situation in Europe where tests are supposed to be mutually recognised but not in every case (like Germany).
There isn't even a standard way of indicating language level in licences.
Such a simple thing like a language test has become more difficult to organise than a 737 type rating.